1. Field of the Invention
This invention relates to a wobble signal detecting circuit for an optical disk reproducing device for an optical disk with a track including a portion snaking at a specific period.
2. Description of the Related Art
Recent DVDs (Digital Versatile Disc) include a reproduction-only media and recordable media. For the reproduction-only media, there is a copy control technology adopted in the DVD standard of CPPM (Content Protection for Prerecorded Media).
According to this standard, the optical disk such as DVD includes a wobble that is a guide track groove snaking at a prescribed amplitude and period over the entire disk. In reproducing DVD-AUDIO, whether or not there is the wobble is determined. If there is no wobble, reproduction must not be performed. In reproducing DVD-AUDIO, therefore, the wobble signal must be detected.
Various detecting circuits have been proposed for detecting the wobble signal (see e.g. JP-A-2001-118243).
FIG. 8 shows a general configuration of a conventional wobble signal detecting circuit.
A wobble signal detecting circuit 100 detects the wobble signal using the outputs from an optical detector 101 provided on an optical pick-up. Specifically, the optical detector 101 has a first portion A, a second portion B, a third portion C and a fourth portion D which are four square parts into which the entire area of the optical detector 101 is divided. In this case, it should be noted that the extending direction of a dividing line of the first portion A and second portion B accords with the tangent direction of a guiding track not shown.
The first portion A and fourth portion D are connected to a first adder 102, whereas the second portion B and third portion C are connected to a second adder 103. Further, the output from the first adder 102 and the output from the second adder 103 are connected to a subtracter 104. The output from the subtracter 104 is connected to a band-pass filter (BPF). The band-pass filter 105 is provided to remove the noise component other than the wobble frequency. The pass bandwidth of the band-pass filter 105, therefore, is set so that its central frequency is approximately equal to the wobble frequency. The output from the band-pass filter 105 is connected to a frequency detecting circuit 107 through a binary-digitizing circuit 106. The output from the frequency detecting circuit 107 is supplied, as a wobble detecting signal (wobble frequency component), to a system controller 108.
More specifically, on the basis of the signals (A+D) and (B+C) supplied to the subtracter 104, a push-pull signal (A+D)−(B+C) is created. The noise component of this push-pull signal is removed by the band-pass filter 105. The frequency component of the wobble signal is extracted by the binary-digitizing circuit 106 and the frequency detecting circuit 107.
FIG. 9 shows the manner of extracting the wobble signal by the band-pass filter 105. FIG. 9A is a spectrum diagram of the entire measured signal including a wobble signal S11. FIG. 9B is a spectrum diagram showing the pass bandwidth of the band-pass filter 105. The center frequency thereof is set to be approximately equal to the frequency of the wobble signal (in this embodiment, 150 kHz). FIG. 9C is a spectrum diagram of the signal after having passed through the band-pass filter 105, which shows the status when the wobble signal has been extracted.